The invention relates to the field of acoustics. More specifically, it is aimed at fixed devices for attenuating the noise generated by moving sources, especially such as transportation means in general and aircraft or land transport in particular.
The invention constitutes an improvement of the device described in the Applicant""s patent EP 0,787,340.
The Applicant has described in the aforementioned patent a device for attenuating the intensity of sound which operates on the principle of the emission of an antinoise wave generated on the basis of information coming from sensors and emitted by electroacoustic sources placed in such a way that the antinoise waves combine with the noise waves that they admit as envelope.
The principles described in that patent remain valid for the present improvement so that said document is cited here as a reference, and its content will therefore not be explained in detail below.
In the embodiments illustrated in that document, the various antinoise sources are combined by subassemblies mounted on vertical masts, that is to say in a direction approximately perpendicular to the mean direction of incidence of the noise waves.
In that document, the various masts are placed near the region to be protected and preferably around the periphery of the region to be protected.
However, it has been found that the separation of the masts as described in that document does not allow the sound waves having a relatively high frequency, and especially greater than 500 Hz (hertz), to be sufficiently attenuated.
One problem that the invention aims to solve is that of the effective attenuation of sound waves lying in a range up to one kilohertz, or even up to 2 kHz (kilohertz).
The invention therefore relates to an active device for attenuating the intensity of sound in a defined region, by the emission of antinoise waves, of the type comprising:
a set of sensors capable of determining the signals and the directions of the waves emitted by remote noise sources;
means for processing the signals coming from said sensors and for generating signals corresponding to the antinoise waves;
a set of electroacoustic sources, said sources being installed in the space close to the region to be protected and connected to said processing means and being capable of emitting antinoise waves in the same direction and in the same sense as the incident waves, the sensors and the electroacoustic sources being placed in such a way that the incident waves reach the sensors beforehand.
This device is distinguished in that the electroacoustic sources are arranged on a continuous surface and in a uniform lattice.
In other words, the invention consists in combining the various sources in such a way that they constitute a lattice close enough to allow attenuation of the high-frequency waves, that is to say in the application to the treatment of sound waves of the order of one kilohertz in frequency.
Thus, according to one characteristic of the invention, the sources are spaced apart from the region by a distance of between one and two meters.
It may be readily appreciated that the use of masts as described in the aforementioned document would be completely unrealistic for covering a frequency range going up to one kilohertz, since it would result in much too high a mast density on the ground.
This is because, according to one theory on the operation of active screens, it seems that the monolayer continuous screening effect is limited in the frequency range because of the discrete distribution of these sources over the surface. What is involved is a low-pass phenomenon whose cutoff frequency is: f0=xcex1co/a, where:
a denotes the characteristic dimension of the source lattice cell;
xcex1 is a parameter slightly greater than 1, characteristic of the geometrical shape of the cell; and
c0 is the speed of sound.
Above this cutoff frequency, the incident waves are no longer only reflected by the screen but also diffracted upstream and downstream of the screen, with the effect of inducing a pressure level twice the level of the incident wave, which then makes said screen not only inoperative but also disruptive.
The choice of a sufficiently close lattice cell, of the order of magnitude of half a meter, makes it possible to obtain a cutoff frequency of the order of one kilohertz encompassing most of the power spectrum of the sound wave from an airplane, for example.
Thus, the various sources are arranged over surfaces which may be formed by a lattice which is itself raised up, placed above the region to be covered or above the buildings which adjoin said region.
The term xe2x80x9ccontinuous surfacexe2x80x9d should be understood to mean a surface which exhibits geometrical regularity such that all the sources may be regarded, with respect to a noise wave, as equivalent in their contribution to the attenuation, to within the effect of their orientation.
Such surfaces may be plane, or else, for example, belong to the family of quadrics, especially cylinders.
In practice, it has been found that a hexagonal lattice allows the sources to be most compact and therefore achieves the best coverage within a frequency band for the same source density.
According to another characteristic of the invention, the device according to the invention comprises several sets of electroacoustic sources arranged over several surfaces offset one with respect to the other by translation normal to their surface, so as to form multilayer complex electroacoustic sources, thereby increasing their transverse spacing for the same bandwith.
Thus, when the loudspeakers, which form the electroacoustic sources, are combined on surfaces which are close together and more or else parallel, these combinations of loudspeakers have the effect of one loudspeaker of larger cross section, without occupying the area thereof.
This is because a single loudspeaker of identical working area would occupy too high a proportion of the lattice, which in turn would reduce the visual transparency of the screen.
In particular configurations, several devices may be combined in such a way that these devices are juxtaposed beside one another in the space of the region to be protected in order to cover one particular geometrical region such as, for example, a crossroad. These devices may be combined so as to be continuous with surfaces of the same type forming passive screens, especially glazed structures, for architectural and functional reasons.
The various screens are driven by a microphonic pickup system located closely upstream of the screen. This noise wave pickup system has the ability to separate and characterize these waves, in terms of direction and of signal respectively, so as to allow the antinoise sources to counteract them additively.
In the case of a single noise source, all the echoes carry practically the same signal, namely that of the direct wave. This is therefore the signal of the first wave picked up with an amplitude factor and a time delay.
The control means are capable, using the appropriate algorithms, of extracting the common reference signal, together with the amplitude and delay parameters specific to each echo signal, from a set or from a base of microphonic sensors placed upstream of the screens.
The minimum number of sensors to be used in the microphonic base is at least equal to the number of signals to be discriminated, but, in practice, this number is greater in order to overcome the effect of parasitic noise of nearby origin.
In more complex situations, the noise sources are multiple and independent sources such as, for example, in the case of noise generated by land transport means such as vehicles, automobiles or trucks.
In this case, the number of specific signals, which are independent sources, is more than about ten. Many complementary directional microphonic bases will then be used, these preferably being arranged as close as possible to the sources.
For example, the microphonic bases may be arranged along the highway or along the railroad track for selective acquisition, by proximity of the various reference signals specific to the independent sources, such as wheel trains, bogies and aerodynamic boundary layers.
Consequently, the separation of the various noise waves at the screen is facilitated by prior knowledge.
However, in this case the signals which propagate from the microphonic base to the screens are subject to the vagaries of the atmospheric propagation of sound, which must be taken into account in the algorithms for separating the signals in the microphonic base close to the screen.
In all cases, the algorithm principles used for signal selection require great accuracy. This accuracy of the algorithms is determined by the overall accuracy of reconstruction of the antinoise waves, which is evaluated in the following manner.
Assuming that the active screen is intended to oppose a noise wave of amplitude n(t), by generating an antinoise wave of amplitude an(t), the amplitude of the residual noise is e(t)=a(t)xe2x88x92an(t). The quadratic norm, or its energy evaluated over the time characteristic of the auditory perception of the noise signals (about one tenth of a second) is as follows: {overscore (e2)}={overscore (n2)}xe2x88x922{overscore (an.n)}+{overscore (an2)}, where the bar above the symbols denotes the time-averaging effect.
The attenuation factor Att={overscore (e2)}/{overscore (n2)} is expressed by the following formula:
Att=2r(1+M/2)+(1xe2x88x92r)M2/4
in which:
r is the defect coefficient at the unit of correlation of the n(t) and an(t) signals, which is given by the following formula:
1xe2x88x92r={overscore (a.an2)}/{overscore (n2.an2)}; and
xe2x80x83M is the ratio of the energies, according to the following formula:
1+M={overscore (an2)}/{overscore (n2)}.
In order to obtain an attenuation factor of the order of 20 decibels, it is therefore necessary for the antinoise signal/noise signal correlation coefficient to be 0.995, which value demonstrates the very great similarity to be obtained between broadband noise signals.
This value means overall that the various elements involved in the attenuation device must have a quadratic accuracy of the order of 2xc3x9710xe2x88x923, not achieved in the field of standard sound reproduction.